Well we have covered the refracting telescope, binoculars and the reflecting telescope. Now let's move onto the most complicated telescope of the three.
Catadioptric telescopes make use of the best of both the refractor and reflecting telescopes. They use a combination of mirrors and a corrector plate lens to provide a physically short tube, but with a much longer focal length. The two most popular types of telescope to come under this category are the "Schmidt Cassegrain" and the "Maksutov Cassegrain"
To understand how this somewhat curious design works, it is easier, for the time being, to think of them as a short Newtonian reflector with a type of lens placed across the front of the tube. Also for now I will refer to both types as CATs. Not to confuse this CAT with the furry variety.
The CAT has a tube with a primary mirror at the bottom, just as the reflector and a secondary mirror near the top of the tube. The big difference being the secondary mirror. You will hopefully remember from the reflector that we sometimes refered to the secondary mirror as the flat. However with this design of scope the secondary mirror is not flat at all. It has a convex shape, not unlike a lens, but it is a mirror. The primary mirror has the same type of parabolic shape as the Newtonian but with a much deeper dish. This gives the mirror a much shorter focal length.
Light enters and travels down the tube until it encounters the primary mirror. It is then reflected back up the tube where it meets the secondary mirror. The light is then reflected back down the tube again, only this time the rays have been focused into a cone shape and through a hole in the centre of the primary mirror. Here they pass into the focuser and eyepiece at the rear of the scope.
The curved secondary mirror is held in place by a glass plate that fits right over the aperture of the tube. This plate also has a distinct shape to it and is known as the corrector plate. Although a CAT is a very clever design, the very short focal length primary mirror, that it uses, causes some problems. For a start the mirror may suffer from some spherical aberration, because the parabolic shape lies mostly in the centre of the mirror, right where there is a hole.
Some designs do not use a parabolic mirror at all. Instead they use a spherical mirror and rely on some divergence of the light rays caused by the corrector plate. This provides the same results as a parabolic mirror. Very short focal length mirrors can also suffer from a problem known as "coma". This can be seen as giving stars an almost comet look about them when they are close to the edge of the field of view.
The corrector plate is shaped to provide the same aberrations as the mirror, but in reverse. And just as the the refractor's objective lens uses two (or more) elements, to cancel out each others aberrations, the corrector plate does the same for the primary mirror. In the case of the Schmidt Cassegrain this corrector plate may appear to be flat, but it will have been figured.
The convex shape of the secondary mirror causes the light rays to spread somewhat as they travel back down the tube. This has the effect folding the light path and visually extending the length. Although the physical length of the telescope seems quite short. It is not unusual for a CAT to have a focal length that is more than three times its physical length. This makes the telescope much lighter and considerably more compact than both the refractor and the reflector.
The main difference between the Schmidt Cassegrain telescope and the Maksutov Cassegrain telescope is in the shape of the corrector plate. The Maksutov has a corrector plate with a much deeper dish shape to it. The Maksutov corrector plate is said to have a meniscus contour.
One important point to remember about a CAT telescope is they tend to dispense with a focuser, in the normal sense. A CAT telescope instead features a mechanism that moves the primary mirror to focus the telescope. This has pros and cons, but allows the telescope to achieve a much broader focusing range to accommodate many different devices, cameras etc.
Both the Schmidt Cassegrain and the Maksutov Cassegrain telescopes fall into the category known as catadioptric telescopes. They make use of mirrors, in a similar way to the Newtonian and a special type of lens known as a corrector plate, which is full aperture. They feature a folded light path, that is to say that the light is reflected up and down the tube several times. This results in a physically short tube with a much longer focal length.
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